Immunotherapy with PD-1 or PD-L1 blockade is changing the landscape of cancer therapy. Nowhere is this more evident than in melanoma, where PD-1/L1 blockade induces sustained tumor responses in a significant number of patients leading to the first approval of the anti-PD-1 antibody pembrolizumab by the FDA. We have been studying response and resistance to this therapy and we are now in the position to propose mechanistic studies aimed at providing a molecular understanding of response, innate and acquired resistance to PD-1 blockade. In Aim 1 we propose to characterize the T cell responses unleashed by blocking PD-1 in patient-derived samples. This will include phenotypic and functional studies in tumor infiltrating lymphocytes (TIL), as well as analyses of the fine antigen specificity of the therapeutic T cells. With the long term follow up of the initial cohortsof patients treated with the PD-1 blocking antibody pembrolizumab we are now seeing cases of delayed tumor progression after a long period of objective tumor response. In Aim 2 we will study the potential mechanisms of these acquired resistance cases by analyzing changes in T cell function, antigen processing machinery and neoepitope mutational changes. We want to better understand response and resistance to PD-1/L1 blockade in an animal model. Therefore, in Aim 3 we propose generating a panel of BRAF-driven murine melanomas through carcinogenic exposure and characterize their response and resistance to anti-PD-1 therapy. These models will also be used to test combinatorial approaches to improve on the antitumor activity of PD-1/L1 blocking therapy. In conclusion, our proposal is based on hypothesis-driven bench-to-bedside-and-back mechanistic studies with the goal of patient-centric advances in cancer immunotherapy.